Earth Electrode:
- Any wire, rod, pipe, plate or an array of conductors, embedded in ground horizontally or vertically is known as the earth electrode.
Earth Resistance:
- The resistance offered by the earth electrode to the flow of current into the ground is known as resistance to earth.
- Mathematically, earth resistance given as V/I where V is the measured Voltage between the electrode and the voltage spike and I is the injected current during the earth resistance measurement through the electrode.
- Following values of earth resistance values will give satisfactory results.
- Large Power stations = 0.5 ohm
- Major Power stations = 1.0 ohm
- Small Substations = 2.0 ohm
- In all other case = 8.0 ohm
Electrical Current Tolerable limits for human body :
- The effect of electric current on a human body depends upon the current, time duration and frequency.
- The body can tolerate higher ( approximately four to five times ) direct current then alternating current at 50Hz.
- For Dc system the current will flow only if the system is earthed, or if simultaneous contact is made with two points at different potential.
- The effects of electrocution are body burn and sometimes stoppages of heart.
- The current range of 1.6 mA, do not impain person ability. It is also known as "let go current ".
- The current range of 9-25mA, may be painful and affect the person ability. this range of current make heart breathing difficult.
- The currents higher than 60mA , dangerous for human body.
- The permissible current and duration should be less than that required for ventricular fibrillation of the heart and is calculated by the following equation,
For up to 3 sec.
Ib=0.116/root of t [persons of 50kg weight]
Ib = 0.157/root of t [persons of 70 kg weight]
where Ib is the permissible body current in amperes and t is the duration of current in sec. the value of 0.116 is constant.
- This constant is based on experiments conducted by different people and therefore it varies.
- The resistance of the human body is assumed to be 1000 ohms for current passing from hand to foot.
- Human skin is very thin and can be punctured quickly and therefore protective gear must be worn while working on electrical equipment.
- It is also advisable to wear full overall made from cotton which provide good resistance.
- In addition to reducing burns, a full sleeved shirt or overall allows others to grab the electrocuted person and pull away without endangering themselves.
- Rubber gloves must be tested regularly for any sign of punctuate and for its insulation strength.
- It is always a good practice to stand on a rubber mat while operating pn a switchboard.